A fuel cell is an electrochemical cell which consumes fuel and an oxidant on a continuous basis to generate electrical energy. The fuel is consumed at an anode and the oxidant at a cathode. The anode and cathode are placed in electrochemical communication by an electrolyte. One typical fuel cell employs a phosphoric acid electrolyte. The phosphoric acid fuel cell uses air to provide oxygen as an oxidant to the cathode and uses a hydrogen rich stream to provide hydrogen as a fuel to the anode. After passing through the cell, the depleted air and fuel streams are vented from the system on a continuous basis.
A typical fuel cell power plant comprises one or more stacks of fuel cells, the cells within each stack being connected electrically in series to raise the voltage potential of the stack. A stack may be connected in parallel with other stacks to increase the current generating capability of the power plant. Depending upon the size of the power plant, a stack of fuel cells may comprise a half dozen cells or less, or as many as several hundred cells. Air and fuel are usually fed to the cells by one or more manifolds per stack. Examples of typical fuel cell power plants are shown in U.S. Pat. No. 3,585,078 issued to Sederquist et al. entitled "Method Of Reformer Fuel Flow Control", U.S. Pat. No. 3,976,507 issued to Bloomfield entitled "Pressurized Fuel Cell Power Plant With Single Reacting Gas Stream"; and U.S. Pat. No. 4,202,933 issued to Riser, et al. entitled "Method For Reducing Fuel Cell Output Voltage To Permit Low Power Operation". The information contained in these patents is incorporated herein by reference.
Fuel cell components are designed to operate within a band of predetermined voltages. Voltages above the predetermined maximum are avoided in acid cells because excessive voltages may damage internal equipment and cause excessively fast corrosion of components such as the cathode. In all fuel cells, such high voltages result in low power densities and uneconomical operation of the power plant. Voltages below a predetermined minimum are avoided because such low voltages adversely affect the efficiency of the fuel cell causing the fuel cell to require a larger amount of fuel for a given amount of power.
As shown in FIG. 2, fuel cells typically produce electrical energy with a voltage characteristic that decreases as current increases. This graphical representation of voltage and current is often referred to as the voltage characteristic of the fuel cell. The voltage drops with increasing current because of ohmic and polarization losses. In addition, there is a voltage loss with time due to the slow deterioration of catalysts used at the anode and cathode of the fuel cell.
This decreasing voltage characteristic of fuel cells causes difficulties in directly coupling the fuel cell to an electrochemical cell to perform an electrochemical process. Examples of electrochemical cells that use electrical energy to produce a chemical product such as chlorine or caustic alkalis are shown in U.S. Pat. No. 4,031,000 issued to Nakamura et al. entitled "Diaphragm For Electrolytic Production Of Caustic Alkali", in U.S. Pat. No. 4,272,337 issued to Darlington entitled "Solid Polymer Electrolyte Chlor-Alkali Electrolysis Cell" and in U.S. Pat. No. 4,273,626 entitled "Electrolyte Series Flow In Electrolytic Chlor-Alkali Cells", the information in which is incorporated herein by reference.
These electrochemical processes typically employ an electrochemical cell having a voltage characteristic which is opposite in nature to the voltage characteristic of the fuel cell. In these cells, the production of the saleable product is directly proportional to the flow of current through the cells. As shown in FIG. 2, increasing voltages are required as the flow of electrical current is increased through the electrochemical cell to produce more product. The increasing voltages are needed to overcome ohmic and polarization losses in the electrochemical cell and other losses which are similar to the losses occuring in a fuel cell. Thus, as the current and power consumption is increased in the electrochemical cell to produce more chemical product at an efficient operating point, the voltage increases. As the power supplied by the fuel cell increases to meet this demand, the operating voltage of the individual cells is decreased.
Accordingly, scientists and engineers are seeking a way to match the performance of a fuel cell to an electrochemical cell to combine the two cells in a cycle and yet to allow the fuel cell to operate at a voltage most beneficial to the fuel cell and the electrochemical cell to operate at a voltage most beneficial to the electrochemical cell.